The significance of these latter factors is most easily explained by referring to common irrigation or watering tubes. These are usually cylindrical tubes in which a series of dispensing holes or openings are located so as to be spaced from one another in a series extending along the length of the tube. The walls in such tubes are normally as thin as possible without being so weak as to be apt to break under the internal pressures which such tubes are intended to contain as they are used. Such pressures can be referred to as "rated" pressures. While the use of such thin walls is desirable in minimizing the amount of material required to form a tube of a specific internal diameter and length it results in such tubes having comparatively thin walls.
This latter is considered disadvantageous because it tends to make it exceedingly difficult to form the holes in such tubes so that they are small enough to provide sufficient resistance to water flow so that the amount of water dispensed through such a hole during normal conditions of use will be as limited as may be desired for some agricultural and related applications. It is considered a practical impossibility to increase such resistance to flow by making such holes smaller than small laser drilled or burnt holes as are found in current irrigation tubes because it may not be possible to form such smaller holes and, if it is possible, it probably is not practical or economical to form them.
As a consequence of the effective limit to which the size of a dispensing hole in an irrigation tubing can be reduced a number of efforts have been made to develop specialized tube constructions and related devices used with irrigation tubing to limit the amount of water capable of being dispensed from an irrigation tube. Fortunately an understanding of the present invention does not require a consideration of all of such expedients. Many of them are based upon the principle that the longer the passage water has to pass through to be dispensed the greater the pressure drop across the ends of the passage and the less the amount of water emitted. This is related to the fact that the longer a piece of pipe is the greater the pressure required to achieve a specific flow through the pipe.
In the irrigation field such comparatively long passages have been achieved in a number of different ways. In some cases housings having passages which extend back and forth between the sides of the housings are attached directly to an irrigation tube so that all water dispensed from the tube has to flow through such a housing. In other cases tubes have been formed so that opposed edges or surfaces of a strip of material used to form these tubes have been secured together to define comparatively long passages to retard the flow of water. While expedients of this type are considered effective it is not considered that they are as economic as other constructions in which small conventional cylindrical or specialized holes leading through the wall of a tube are used to constrict water flow.
Unfortunately the amount which even the smallest holes capable of being created in the wall of a watering or irrigation tube will not provide enough resistance to fluid flow so as to restrict such flow to the very limited amounts desired for many applications. Efforts to increase such resistance to flow by slanting the holes in accordance with the ancient method employed during Roman civilization to influence the flow from an irrigation canal are not normally satisfactory with present day plastic or polymer tubes because the walls of such tubes are so thin that there is simply not space enough to slant a dispensing hole so as to gain any significant length of the hole.
Common plastic or polymer irrigation tubes as used at the present also can be undesirable in some applications for another, quite different reason. In some applications it is desired to be able to orient the dispensing holes in such tubes so that they face a specific direction. Thus, in some cases it would be desirable to hold an irrigation tube used to dispense limited quantities of water so that the water would not tend to run down the length of the tube as it seeps from the holes in such a tube but instead would tend to drip from the tube adjacent to such holes onto the ground adjacent to such holes. Although cylindrical irrigation tubes can be held in a desired orientation it is rather difficult to accomplish this result.